1. Field of the Invention
This invention relates to a method of recognizing an irradiation field on a radiation image, and an apparatus for carrying out the method. This invention also relates to a blackening processing method, and an apparatus for carrying out the blackening processing method.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image having good image quality by use of the processed image signal have heretofore been known in various fields. For example, as such techniques, the applicant proposed various radiation image recording and reproducing systems which use stimulable phosphor sheets.
When a radiation image of an object, such as a living body, is recorded on a recording medium, such as X-ray film or a stimulable phosphor sheet, it is desirable that adverse effects of radiation upon the living body can be kept as small as possible. Also, if object portions not related to a diagnosis, or the like, are exposed to radiation, the radiation will be scattered by such portions to the portion that is related to a diagnosis, or the like, and the image quality will be adversely affected by the scattered radiation. Therefore, when a radiation image is recorded on the recording medium, a collimation plate made from lead, or the like, is often used in order to limit the irradiation field to an area smaller than the overall recording region of the recording medium, such that radiation may be irradiated only to that portion of the object, the image of which is to be used.
In cases where a radiation image is recorded on a recording medium, such as a stimulable phosphor sheet, by using a collimation plate, an image of an object, or the like, is recorded in a region (i.e., an irradiation field) inward from the aperture contour of the irradiation field stop. Also, a region outward from the aperture contour of the irradiation field stop (i.e., a region outside of the irradiation field) is not exposed to the radiation. Therefore, an irradiation field contour on the image, which contour corresponds to the aperture contour of the irradiation field stop, constitutes edge lines in the image.
In cases where an image signal is detected from the recording medium, on which an image has been recorded within only the irradiation field, and image processing is carried out on the image signal, the image processing, such as gradation processing, may be carried out on only the image signal components of the image signal, which correspond to the region inside of the irradiation field. In this manner, the amount of the processing can be reduced markedly, the load of the processing can be kept small, and the processing speed can be kept high.
The region outside of the irradiation field is not exposed to the radiation. Therefore, in cases where the image is a negative image recorded on medical X-ray film, the image density of the region outside of the irradiation field becomes approximately lowest on the image. By way of example, when the medical X-ray film is set on a viewing screen and the transmission image with light produced by a fluorescent lamp is seen, the region having the lowest level of image density becomes the very bright region. Therefore, in particular, the portion of the irradiation field, which portion is close to the region outside of the irradiation field, cannot be seen clearly due to dazzling effects of the bright region outside of the irradiation field. Also, in cases where an electric image signal is detected from the recording medium, on which an image has been recorded within only the irradiation field, and a visible image is reproduced from the image signal and displayed on an image displaying device, such as a cathode ray tube (CRT) display device, the region outside of the irradiation field has the highest level of luminance, and therefore the image within the irradiation field cannot be seen clearly.
Accordingly, in the radiation image recording and reproducing systems, a process for forcibly replacing the image signal values, which correspond to the region outside of the irradiation field, by image signal values representing the highest level of image density (or the lowest level of luminance) is carried out. The process is ordinarily referred to as the blackening process. It is important that the irradiation field contour, which serves as the spatial reference for the blackening process, can be recognized accurately.
Specifically, if a contour, which is smaller than the correct irradiation field contour (i.e., which is positioned more inward than the correct irradiation field contour), is recognized as the irradiation field contour by mistake, the image portion, which is to be used as the one within the irradiation field, will be subjected to the blackening process as being the one outside of the irradiation field, and therefore the image information at the image portion cannot be seen on the image obtained from the blackening process. Also, if a contour, which is larger than the correct irradiation field contour (i.e., which is positioned more outward than the correct irradiation field contour), is recognized as the irradiation field contour by mistake, the image portion, which has the lowest level of image density (or the highest level of luminance), will remain unblackened, and the effects to be obtained from the blackening process cannot be obtained.
Accordingly, various techniques for accurately carrying out the process for recognizing the irradiation field (contour) have been proposed.
For example, techniques have been proposed, wherein an irradiation field contour is determined by utilizing the characteristics in that the irradiation field contour constitutes the edge lines in the image, at which the image density changes sharply, and finding a portion, at which the value of the image signal changes sharply. One of such techniques for determining the edge lines has been proposed in U.S. Pat. No. 4,967,079. With the technique proposed in U.S. Pat. No. 4,967,079, the edge lines are determined by (a) setting a plurality of radial straight lines, which extend from a predetermined point in the image (e.g., a center point in the image) toward ends of the image, (b) detecting an edge candidate point, at which the different in image signal value is large, from the image signal values corresponding to positions along each of the radial straight lines, a plurality of edge candidate points being thereby detected with respect to the plurality of the radial straight lines, and (c) determining the edge lines in accordance with the thus detected edge candidate points.
However, with the aforesaid technique for recognizing the edge lines, i.e. the irradiation field contour, in accordance with the edge candidate points having been obtained with respect to the radial directions, an irradiation field contour, which coincides with the actual irradiation field contour, cannot necessarily be obtained. In such cases, the problems occur in that an image portion, which is to be used, is blackened with the blackening process, or in that an image portion, which has the lowest level of image density and is to be blackened, remains unblackened.
The primary object of the present invention is to provide a method of recognizing an irradiation field on a radiation image, wherein the accuracy, with which the irradiation field is recognized, is kept to be higher than the accuracy of a conventional technique for recognizing an irradiation field.
Another object of the present invention is to provide an apparatus for carrying out the method of recognizing an irradiation field on a radiation image.
A further object of the present invention is to provide a blackening processing method for a radiation image, wherein the accuracy, with which a blackening process is carried out, is kept to be higher than the accuracy of a conventional technique for the blackening process.
A still further object of the present invention is to provide an apparatus for carrying out the blackening processing method for a radiation image.
A method and apparatus for recognizing an irradiation field on a radiation image in accordance with the present invention are characterized by detecting edge candidate points with respect to radial straight lines, which have been set with respect to a predetermined point lying in the image, detecting a predetermined number of reference candidate lines by the utilization of Hough transform with respect to the edge candidate points, and determining that the region surrounded by the reference candidate lines is the irradiation field.
Specifically, the present invention provides a method of recognizing an irradiation field on a radiation image, in which an image signal representing a radiation image is obtained, the radiation image having been recorded by use of a collimation plate and having an irradiation field thereon, and a process for recognizing the irradiation field is carried out, the irradiation field being recognized in accordance with the image signal, the method comprising the steps of:
i) setting a plurality of radial straight lines extending from a predetermined point, which lies within the irradiation field, toward ends of the radiation image,
ii) detecting an edge candidate point with respect to each of the set radial straight lines and in accordance with values of the image signal, which correspond to positions located along the radial straight line, the edge candidate point being considered to constitute a contour of the irradiation field on the radial straight line, a plurality of edge candidate points being thereby detected with respect to the plurality of the radial straight lines,
iii) detecting a curved line, which may be represented by Formula (1), with respect to each of the edge candidate points:
xcfx81=xi cos xcex8+yi sin xcex8xe2x80x83xe2x80x83(1)
wherein coordinates of each of the edge candidate points on an orthogonal coordinate system having been set in the plane of the radiation image are represented by (xi, yi), and xi and yi are taken as fixed values, a plurality of curved lines being thereby found with respect to the plurality of the edge candidate points,
iv) detecting points of intersection (xcfx81j, xcex8j), at which the detected curved lines intersect with one another, the number of the curved lines, which intersect with one another at each of the points of intersection (xcfx81j, xcex8j), being then counted with respect to each of the points of intersection (xcfx81j, xcex8j),
v) extracting a predetermined number of the points of intersection (xcfx81j, xcex8j) in the order of decreasing count value,
vi) detecting a reference candidate line on the orthogonal coordinate system, the reference candidate line corresponding to each of the extracted points of intersection (xcfx81j, xcex8j) and being defined by Formula (2):
xcfx81j=x cos xcex8j+y sin xcex8jxe2x80x83xe2x80x83(2)
a plurality of the reference candidate lines, which correspond to the extracted points of intersection (xcfx81j, xcex8j), being thereby detected, and
vii) determining that a region surrounded by the plurality of the reference candidate lines defined by Formula (2) is the irradiation field.
By way of example, as the predetermined point, from which the radial straight lines extend, the physical center point in the image may be employed. This is because it is not popular that the region outside of the irradiation field is located at the center portion of the image. However, in the method of recognizing an irradiation field on a radiation image in accordance with the present invention (and in the apparatus for carrying out the method, which will be described later), the predetermined point need not necessarily be set at the center point in the image and may be set at one of the other points in the image.
The detection of the edge candidate point with respect to each radial straight line may be carried out in the manner described below. Specifically, a calculation may be made to find the difference between the image signal values, which correspond to two adjacent picture elements lying on the radial straight line, and a plurality of such differences may be calculated successively. Two adjacent picture elements, which are associated with the largest difference, may thereby be detected, and one of the two adjacent picture elements, which has a smaller image signal value than the other, may be detected as the edge candidate point.
As the radiation image, one of various kinds of images may be employed. For example, the radiation image may be an image (i.e., an original image), which has been read out from the recording medium. Alternatively, the radiation image may be an image having been obtained from a predetermined normalizing process, which is carried out such that the image density, the gradation, or the like, may fall within a predetermined range. As another alternative, the radiation image may be a reduced image, which is obtained by thinning out the picture elements in the original image. As a further alternative, the radiation image may be a preliminary read-out image having been obtained from a preliminary read-out operation, which is carried out before a final read-out operation for obtaining the original image is carried out and which is carried out for picture elements that are larger than those of the final read-out operation.
As described above, it is determined that the region surrounded by the plurality of the reference candidate lines defined by Formula (2) is the irradiation field. At this time, it may occur that two or more such closed regions are formed. In such cases, the areas of the closed regions other than a single certain closed region are markedly small. Therefore, in such cases, a single irradiation field can be determined by calculating the areas of the closed regions and comparing the calculated areas with one another.
The aforesaid predetermined number, which determines the number of the extracted reference candidate lines, should be equal to at least the number of the linear sides, which constitute the irradiation field contour.
The method of recognizing an irradiation field on a radiation image in accordance with the present invention may be modified in the manner described below. Specifically, evaluation utilizing a predetermined evaluation function may be made with respect to each of the reference candidate lines defined by Formula (2),
predetermined candidate lines may be detected in accordance with the results of the evaluation, the predetermined candidate lines being detected from the reference candidate lines or in lieu of the reference candidate lines, and
it may be determined that a region surrounded by the predetermined candidate lines, in lieu of the reference candidate lines, is the irradiation field.
In the modification described above, the evaluation utilizing the predetermined evaluation function may be made by making a judgment for each of the reference candidate lines defined by Formula (2) as to whether a predetermined representative point, which has been set previously as a point lying on the irradiation field contour, lies or does not lie on the reference candidate line, and
the predetermined candidate lines may be the reference candidate lines, which are among the reference candidate lines defined by Formula (2) and for which it has been judged that the corresponding predetermined representative points lie thereon.
The term xe2x80x9cpredetermined representative point having been set previously as a point lying on a irradiation field contourxe2x80x9d as used herein means the point, which has a strong probability of lying on the irradiation field contour on the actual image. For example, the predetermined representative point may be the point, at which the aforesaid difference between the image signal values corresponding to two adjacent picture elements takes a markedly large value.
In the modification described above, alternatively, the evaluation utilizing the predetermined evaluation function may be made by shifting and/or rotating each of the reference candidate lines, which are defined by Formula (2), within the range of xc2x1m picture elements and xc2x1xcex1 degrees around the reference candidate line, thereby setting {(2m+1)(2xcex1+1)xe2x88x921} number of transformed candidate lines with respect to each of the reference candidate lines, calculating differentiated values with respect to each candidate line, which is among the reference candidate lines and the transformed candidate lines having been set for the reference candidate lines, and calculating a mean value of the differentiated values with respect to each candidate line, the mean value being taken with respect to the direction, along which the candidate line extends, and
the predetermined candidate lines may be a predetermined total number of the reference candidate lines and the transformed candidate lines, which have been selected in the order of decreasing mean value of differentiated values, in lieu of the reference candidate lines defined by Formula (2).
The value of m and the value of xcex1 should preferably be set such that, for example, m=1 (picture element) and xcex1=1/(x2+y2)xc2xd (degree). In this formula, x and y are the values representing the size of the reduced image, which is expressed with the number of picture elements (x picture elements x y picture elements). For example, in cases where the size of the reduced image is 100 picture elements x 80 picture elements, x=100 (picture elements), and y=80 (picture elements). Therefore, in such cases, m=1 (picture element), and xcex1=1/(1002+802)xc2xd=7.8xc3x9710xe2x88x923 (degree).
In cases where the scale of reduction falls within the range of {fraction (1/18)} to 1 (scale of enlargement=1 to 18), the value of the reciprocal of the scale of reduction (i.e., the same value as the scale of enlargement) may be employed as the value of m and the value of xcex1. For example, in cases where the scale of reduction is {fraction (1/9)}, the reciprocal (=9) of {fraction (1/9)} may be employed such that m=9 (picture elements) and xcex1=9 (degrees).
The term xe2x80x9cdifferentiated value with respect to each candidate linexe2x80x9d as used herein for the evaluation function described above means the value of difference between the image signal components representing two picture elements, which stand facing each other with the candidate line intervening therebetween. A large difference value represents that the difference in image density (or in luminance) between the two picture elements is large, and that an edge in the image lies between the two picture elements. As illustrated in FIG. 9A, a plurality of sets of the two picture elements, which stand facing each other with the candidate line intervening therebetween, are located in the direction, along which the candidate line extends. Therefore, a plurality of differentiated values are obtained with respect to the plurality of the sets of the two picture elements, the sets being located in the direction, along which the candidate line extends. The differentiated values having been obtained with respect to the sets of the two picture elements, the sets being located in the direction, along which the candidate line extends, are added to one another, and the thus calculated sum is divided by the number of the sets of the two picture elements. In this manner, the mean value described above is calculated. A large mean value represents a strong probability that the candidate line will be the actual edge in the radiation image, and the edge, which is associated with the largest difference in image density (or in luminance), is the line constituting the irradiation field contour.
In the modification described above, as another alternative, the evaluation utilizing the predetermined evaluation function may be made by shifting and/or rotating each of the reference candidate lines, which are defined by Formula (2), within the range of xc2x1m picture elements and xc2x1xcex1 degrees around the reference candidate line, thereby setting {(2m+1)(2xcex1+1) xe2x88x921} number of transformed candidate lines with respect to each of the reference candidate lines, finding directions of image density gradient vectors with respect to each candidate line, which is among the reference candidate lines and the transformed candidate lines having been set for the reference candidate lines, and calculating an entropy of the directions of image density gradient vectors with respect to each candidate line, the entropy being taken with respect to the direction, along which the candidate line extends, and
the predetermined candidate lines may be a predetermined total number of the reference candidate lines and the transformed candidate lines, which have been selected in the order of increasing entropy, in lieu of the reference candidate lines defined by Formula (2).
The term xe2x80x9cimage density gradient vector with respect to each candidate linexe2x80x9d as used herein for the evaluation function described above means the vector directed from each of picture elements, which are located on one side of the candidate line, toward the direction, in which the gradient of the image density (i.e., the value of difference in image signal value) is largest. An index value representing the direction, to which the image density gradient vector is directed, is calculated. As the index value, for example, the sine value (sin xcex2) of an angle xcex2, which is made between the direction of the image density gradient vector and the direction that is normal to the candidate line, may be employed. As illustrated in FIG. 9B, with respect to the directions of image density gradient vectors from the picture elements located in the direction, along which the candidate line extends, a histogram of the index values is formed. Thereafter, the value of entropy, xe2x88x92xcexa3{(Pi)log Pi}, is calculated from a probability density Pi of the histogram. A small entropy represents that the directions of image density gradient vectors are directed in the same direction. Therefore, there is a strong probability that the candidate line, which is associated with a small entropy, will be the line constituting the actual irradiation field contour on the radiation image.
In cases where the function for calculating the entropy is employed as the evaluation function, the evaluation is made in accordance with whether the directions of image density gradient vectors are or are not directed in the same direction. Therefore, the evaluation function utilizing the entropy can be employed only in cases where the lines constituting the irradiation field contour are the straight lines. The evaluation function utilizing the entropy cannot be employed in cases where the lines constituting the irradiation field contour are circular arc lines or curved lines.
The term xe2x80x9cimage density gradient vectorxe2x80x9d as used herein is not limited to the cases where the image signal is of the type representing the image density values. The term xe2x80x9cimage density gradient vectorxe2x80x9d as used herein broadly embraces the gradient vectors based upon image signals, which represent gray levels and luminous levels, including the image density values and luminance values.
The present invention also provides an apparatus for carrying out the method of recognizing an irradiation field on a radiation image in accordance with the present invention. Specifically, the present invention also provides an apparatus for recognizing an irradiation field on a radiation image, in which an image signal representing a radiation image is obtained, the radiation image having been recorded by use of a collimation plate and having an irradiation field thereon, and a process for recognizing the irradiation field is carried out, the irradiation field being recognized in accordance with the image signal, the apparatus comprising:
i) an edge candidate point detecting means for setting a plurality of radial straight lines extending from a predetermined point, which lies within the irradiation field, toward ends of the radiation image, and detecting an edge candidate point with respect to each of the set radial straight lines and in accordance with values of the image signal, which correspond to positions located along the radial straight line, the edge candidate point being considered to constitute a contour of the irradiation field on the radial straight line, a plurality of edge candidate points being thereby detected with respect to the plurality of the radial straight lines,
ii) a reference candidate line detecting means for:
detecting a curved line, which may be represented by Formula (1), with respect to each of the edge candidate points having been detected by the edge candidate point detecting means:
xe2x80x83xcfx81=xi cos xcex8+yi sin xcex8xe2x80x83xe2x80x83(1)
wherein coordinates of each of the edge candidate points on an orthogonal coordinate system having been set in the plane of the radiation image are represented by (xi, yi), and xi and yi are taken as fixed values, a plurality of curved lines being thereby found with respect to the plurality of the edge candidate points,
detecting points of intersection (xcfx81j, xcex8j), at which the detected curved lines intersect with one another, the number of the curved lines, which intersect with one another at each of the points of intersection (xcfx81j, xcex8j), being then counted with respect to each of the points of intersection (xcfx81j, xcex8j),
extracting a predetermined number of the points of intersection (xcfx81j, xcex8j) in the order of decreasing count value, and
detecting a reference candidate line on the orthogonal coordinate system, the reference candidate line corresponding to each of the extracted points of intersection (xcfx81j, xcex8j) and being defined by Formula (2):
xcfx81j=x cos xcex8j+y sin xcex8jxe2x80x83xe2x80x83(2)
a plurality of the reference candidate lines, which correspond to the extracted points of intersection (xcfx81j, xcex8j), being thereby detected, and
iii) an irradiation field determining means for determining that a region surrounded by the plurality of the reference candidate lines defined by Formula (2), which have been detected by the reference candidate line detecting means, is the irradiation field.
The apparatus for recognizing an irradiation field on a radiation image in accordance with the present invention may be modified in the manner described below. Specifically, the apparatus may further comprise:
an evaluation means for making evaluation, which utilizes a predetermined evaluation function, with respect to each of the reference candidate lines defined by Formula (2), which have been detected by the reference candidate line detecting means, and
a candidate line detecting means for detecting predetermined candidate lines in accordance with the results of the evaluation having been made by the evaluation means, the predetermined candidate lines being detected from the reference candidate lines or in lieu of the reference candidate lines, and
the irradiation field determining means may determine that a region surrounded by the predetermined candidate lines, in lieu of the reference candidate lines, is the irradiation field.
The evaluation means evaluates the eligibility of each of the reference candidate lines defined by Formula (2), which have been detected by the reference candidate line detecting means, for the irradiation field contour or the conformity of each reference candidate line to the irradiation field contour. The candidate line detecting means detects the predetermined candidate lines, which have a high eligibility for the irradiation field contour or a high conformity to the irradiation field contour, i.e. which have at least the same level of probability of constituting the irradiation field contour as the reference candidate lines or a higher level of probability of constituting the irradiation field contour than the reference candidate lines.
In the modification described above, the evaluation means may make a judgment for each of the reference candidate lines defined by Formula (2) as to whether a predetermined representative point, which has been set previously as a point lying on the irradiation field contour, (or a predetermined representative point having been set by the evaluation means itself) lies or does not lie on the reference candidate line, and
the candidate line detecting means may detect, as the predetermined candidate lines, the reference candidate lines, which are among the reference candidate lines defined by Formula (2) and for which it has been judged that the corresponding predetermined representative points lie thereon.
The term xe2x80x9cpredetermined representative point having been set previously as a point lying on a irradiation field contourxe2x80x9d as used herein means the point, which has a strong probability of lying on the irradiation field contour on the actual image. For example, the predetermined representative point may be the point, at which the aforesaid difference between the image signal values corresponding to two adjacent picture elements takes a markedly large value.
In the modification described above, alternatively, the evaluation means may comprise:
a) a transformed candidate line setting means for shifting and/or rotating each of the reference candidate lines, which are defined by Formula (2), within the range of xc2x1m picture elements and xc2x1xcex1 degrees around the reference candidate line, and thereby setting {(2m+1)(2xcex1+1)xe2x88x921} number of-transformed candidate lines with respect to each of the reference candidate lines, and
b) an evaluation value calculating means for calculating differentiated values with respect to each candidate line, which is among the reference candidate lines and the transformed candidate lines having been set for the reference candidate lines by the transformed candidate line setting means, and calculating a mean value of the differentiated values with respect to each candidate line, the mean value being taken with respect to the direction, along which the candidate line extends,.and
the candidate line detecting means may detect, as the predetermined candidate lines, a predetermined number of the candidate lines in the order of decreasing mean value of differentiated values, in lieu of the reference candidate lines defined by Formula (2).
In the modification described above, as another alternative, the evaluation means may comprise:
a) a transformed candidate line setting means for shifting and/or rotating each of the reference candidate lines, which are defined by Formula (2), within the range of xc2x1m picture elements and xc2x1xcex1 degrees around the reference candidate line, and thereby setting {(2m+1)(2xcex1+1)xe2x88x921} number of transformed candidate lines with respect to each of the reference candidate lines, and
b) an evaluation value calculating means for finding directions of image density gradient vectors with respect to each candidate line, which is among the reference candidate lines and the transformed candidate lines having been set for the reference candidate lines by the transformed candidate line setting means, and calculating an entropy of the directions of image density gradient vectors with respect to each candidate line, the entropy being taken with respect to the direction, along which the candidate line extends, and
the candidate line detecting means may detect, as the predetermined candidate lines, a predetermined number of the candidate lines in the order of increasing entropy, in lieu of the reference candidate lines defined by Formula (2).
A blackening processing method and apparatus for a radiation image in accordance with the present invention are characterized by carrying out a blackening process on the region outside of the irradiation field, which has been determined by the method or apparatus for recognizing an irradiation field on a radiation image in accordance with the present invention.
Specifically, the present invention further provides a blackening processing method for a radiation image, in which an image signal representing a radiation image is obtained, the radiation image having been recorded by use of a collimation plate and having an irradiation field thereon, and the image signal is processed such that a region outside of the irradiation field may have approximately the highest level of image density or approximately the lowest level of luminance,
wherein the improvement comprises applying, as the irradiation field, an irradiation field having been determined by a method of recognizing an irradiation field on a radiation image in accordance with the present invention.
The present invention still further provides a blackening processing apparatus for a radiation image, in which an image signal representing a radiation image is obtained, the radiation image having been recorded by use of a collimation plate and having an irradiation field thereon, and the image signal is processed such that a region outside of the irradiation field may have approximately the highest level of image density or approximately the lowest level of luminance,
wherein the improvement comprises applying, as the irradiation field, an irradiation field having been determined by an apparatus for recognizing an irradiation field on a radiation image in accordance with the present invention.
With the method and apparatus for recognizing an irradiation field on a radiation image in accordance with the present invention, the edge candidate points are detected with respect to the radial straight lines, which have been set with respect to the predetermined point lying in the image. Thereafter, the predetermined number of reference candidate lines, which serve as the edge lines, are detected by the utilization of Hough transform with respect to the edge candidate points. It is determined that the region surrounded by the reference candidate lines is the irradiation field.
As described above, with the method and apparatus for recognizing an irradiation field on a radiation image in accordance with the present invention, in the first stage, a plurality of the edge candidate points are detected with respect to the radial straight lines having been set in the image plane. Also, in the second stage, the reference candidate lines, which are the straight line components connecting the plurality of the edge candidate points, are detected by the utilization of Hough transform and in accordance with the plurality of the detected edge candidate points. Therefore, the degree of freedom of the shapes of the irradiation field contours, which can be detected, can be kept higher than with the conventional methods and apparatuses for recognizing an irradiation field, in which an irradiation field contour is detected within a range of shapes having been set previously and in accordance with a plurality of edge candidate points having been detected with the same operation as that in the aforesaid first stage. Accordingly, with the method and apparatus for recognizing an irradiation field on a radiation image in accordance with the present invention, the irradiation field, which markedly conforms to the actual irradiation field contour, can be recognized. Thus the irradiation field can be recognized accurately.
With the blackening processing method and apparatus for a radiation image in accordance with the present invention, the irradiation field contour, which has been detected accurately by the aforesaid method or apparatus for recognizing an irradiation field on a radiation image in accordance with the present invention, is utilized, and the blackening process is carried out with respect to the region outside of the irradiation field. Therefore, the blackening process for the region outside of the irradiation field can be carried out more accurately than with the conventional blackening processing methods and apparatuses.